Continued advancements in the field of optoelectronics have led to the development of liquid crystal microdroplet (LCMD) displays. In this type of display, liquid crystal (LC) material is contained in microdroplets embedded in a solid polymer matrix. Birefringence results from a material having a different index of refraction in different directions. The extraordinary index of refraction (ne) of a liquid crystal molecule is defined as that measured along the long axis of the molecule, and the ordinary index of refraction (no) is measured in a plane perpendicular to the long axis. The dielectric anisotropy of liquid crystals is defined as Δ∈=∈∥−∈⊥, where ∈∥ and ∈⊥, are parallel and perpendicular dielectric constants, respectively. Liquid crystals having a positive dielectric anisotropy (Δ∈>0) are called positive-type liquid crystals, or positive liquid crystals, and liquid crystals having a negative dielectric anisotropy (Δ∈<0) are called negative-type liquid crystals, or negative liquid crystals. The positive liquid crystals orient in the direction of an electric field, whereas the negative liquid crystals orient perpendicular to an electric field. These electro-optical properties of liquid crystals have been widely used in various applications.
One approach to obtaining dispersed microdroplets in a polymer matrix is the method of encapsulating or emulsifying the liquid crystals and suspending the liquid crystals in a film which is polymerized. This approach is described, for example, in U.S. Pat. Nos. 4,435,047; 4,605,284; and 4,707,080. This process includes mixing positive liquid crystals and encapsulating material, in which the liquid crystals are insoluble, and permitting formation of discrete capsules containing the liquid crystals. The emulsion is cast on a substrate, which is precoated with a transparent electrode, such as an indium tin oxide (ITO) coating, to form an encapsulated liquid crystal device.
LCMD displays may also be formed by phase separation of low-molecular weight liquid crystals from a prepolymer or polymer solution to form microdroplets of liquid crystals. This process, described in U.S. Pat. Nos. 4,685,771 and 4,688,900, includes dissolving positive liquid crystals in an uncured resin and then sandwiching the mixture between two substrates which are precoated with transparent electrodes. The resin is then cured so that microdroplets of liquid crystals are formed and uniformly dispersed in the cured resin to form a polymer dispersed liquid crystal device. When an AC voltage is applied between the two transparent electrodes, the positive liquid crystals in microdroplets are oriented and the display is transparent if the refractive index of the polymer matrix (np) is made to equal the ordinary index of liquid crystals (no). The display scatters light in the absence of the electric field, because the directors (vector in the direction of the long axis of the molecules) of the liquid crystals are random and the refractive index of the polymer cannot match the index of the liquid crystals. Nematic liquid crystals having a positive dielectric anisotropy (Δ∈>0), large Δn, which may contain a dichroic dye mixture, can be used to form a transparent and absorbing mode.
LCMD displays may be characterized as normal mode or reverse mode displays. A normal mode display containing liquid crystals is non-transparent (scattering or absorbing) in the absence of an electric field and is transparent in the presence of an applied electric field. A reverse mode display is transparent in the absence of an electric field and is non-transparent (scattering or absorbing) in the presence of an applied electric field.
If an electric field is applied on a LCMD display, liquid crystals in microdroplets are not entirely perpendicular to the substrate. The central part of liquid crystals in the droplets is clear if the refractive index of the polymer matches the ordinary refractive index of the liquid crystals (no). However, liquid crystals near the ends of the microdroplet are strongly bent because they are parallel to the skin of the inner layer. They are, therefore, tilted to the substrate surface, and the refractive index of the liquid crystals cannot match with the refractive indexes of the polymer matrix and inner layer. Therefore, parts of the liquid crystal droplets scatter light and produce haze.
There exists a need for devices that use improved LCMD technologies in projection systems that provide improved diffusion and provide for a wide viewing angle. These functions may be achieved with an improved scattering mechanism using, for example, a non-linear optical matrix system.